This invention relates generally to fiber optical communication systems, and more particularly to a specific structure of a fiber optical communication system for carrying light in two different modes or modal groups. An optical fiber is basically a cylinder made of transparent dielectric materials. A central region, referred to as the core, is surrounded by one or more annular cladding regions, which are usually surrounded by a protective jacket. In step-index fibers, the core has a fixed refractive index and the cladding also has a fixed refractive index less than that of the core. In simple terms, light rays launched into the fiber tend to be guided along it because of repeated total internal reflections at the cylindrical boundary between the core and the cladding materials.
Electromagnetic light fields traveling in the fiber or being scattered by the fiber can be considered as superpositions of simpler field configurations, referred to as the modes of the fiber. The mode concept can be expressed in mathematical terms, but for purposes of explanation of the invention, a discussion in terms of geometric optics is more appropriate. Light rays entering a fiber at relatively low angles of incidence with respect to the longitudinal axis of the fiber are referred to as the lower-order modes of the fiber. Rays entering at higher angles of incidence are referred to as the higher order modes.
Rays entering the fiber at sufficiently low angles of incidence are referred to as "guided," since they are totally internally reflected within the fiber. Other rays may be characterized as "unguided," meaning that most of their energy will pass into the cladding and either be absorbed there or radiated out from the fiber. The invention is concerned with guided lower-order modes and with guided higher-order modes. In communication systems, the different modal groups may be used as different information channels. This is a form of multiplexing, in which a group of lower-order modes is used to carry one information signal and a group of higher-order modes is used to carry another information signal.
In one type of bimodal communication system, the higher-order modes are used not to carry useful information, but rather as an intrusion detection device. When a bend of small radius is applied to an optical fiber, some of the light radiates from the core and may be used to decode the information signal being carried by the fiber. One technique for detecting such intrusion is to use the higher-order modes to carry a signal referred to as a masking signal or a monitor signal. If a fiber is bent to cause radiation of the transmitted light, the higher-order modes will radiate from the fiber core to a much greater degree than the lower-order modes. Monitoring the power ratio between higher-order and lower-order modes at the receiving end of the fiber allows detection of intrusion of this type. Optical fiber links employing this principle are referred to as bimodal intrusion alarmed links.
It is essential in such a bimodal multiplexing arrangement that separate light signals be launched as lower-order and higher-order modal groups. Any crosstalk between the two modal groups will render them ineffective for multiplexing purposes. In the past, the only technique for launching bimodal signals into a single fiber was to launch a lower-order-mode signal from a light source disposed on the central axis of the fiber, and to launch a higher-order-mode signal from a light source displaced from the axis, to provide higher angles of incidence. Unfortunately, in such an arrangement it is difficult to control with any precision the amounts of light that are launched at various angles into the fiber. For example, some of the light from the centrally located source will be launched as higher-order modes in the fiber if the core-cladding interface of the fiber is not a perfect step index.
It will be apparent that these problems are further aggravated if instead of a step-index transmission fiber, a graded index fiber is used. A graded index fiber is one in which there is no well defined boundary between the core and cladding regions of the fiber. Instead, the refractive index of the fiber material gradually decreases from the central axis to the outer radius of the fiber. In one commonly available graded index fiber, the refractive index varies with the radial distance in a parabolic manner. Because of the difficulties of launching bimodal signals into transmission fibers, and adequately maintaining the integrity of two separate modal groups, bimodal optical fiber links have never employed graded index transmission fibers, which would apparently aggravate the launching difficulties. However, step-index fiber for use in bimodal fiber links is relatively expensive to manufacture, mainly because it is made in small quantities compared with other types of fiber.